The present invention relates to a solid electrolytic capacitor and to a manufacturing method thereof. More particularly, the present invention relates to a solid electrolytic capacitor comprising a capacitor element having a substrate comprising a valve-acting metal having a dielectric film on the surface thereof and a solid electrolyte layer on the substrate, the capacitor element having lead wires (lead frames). The solid electrolytic capacitor has excellent strength and heat resistance at the bonding portion between the capacitor element and the lead frames and is highly reliable.
To keep up with recent advancement of digitization or high frequency driving of electric equipment for attaining downsizing or electric power savings, demands for solid electrolytic capacitors having low impedance at high frequency range, high reliability and high capacitance are increasing.
Generally, a solid electrolytic capacitor has a basic structure that includes a plurality of single plate capacitor elements stacked one on another. Each single plate capacitor element has an etched valve-acting metal such as aluminum, tantalum or titanium, having a dielectric oxide film on the surface thereof. It also has a solid electrolyte layer comprising an organic substance layer such as a layer of an electroconductive polymer or an inorganic substance layer such as a layer of a metal oxide on the dielectric oxide film. Furthermore, it has an anode lead wire connected to an anode terminal of valve-acting metal (surface portion of end part where no solid electrolyte is provided) and on the other hand a cathode wire connected to an electroconducting part composed of a solid electrolyte (cathode part). The entire structure is sealed with an insulating resin such as an epoxy resin.
To manufacture a solid electrolytic capacitor having such a structure as described above and also having high reliability, the capacitor must have high strength and excellent heat resistance at the bonded portions between the capacitor element and the lead frames. In particular, chip-type capacitors which are surface mounted on an electronic circuit substrate are designed to have durability against heat at the reflow soldering by using a highly heat resistant material or by constructing the capacitor to enable relaxation of the thermal stress. These solid electrolytes have low resistance but are poor in the recovering activity of dielectric film. Accordingly, the dielectric film may occasionally undergo microscopic destructions due to the thermal stress or the like to increase the leakage current.
Some known structures for bonding between a capacitor element and a lead frame do not have always-sufficient heat resistance. For example, according to the method of JP-A-6-69084, a projecting metal plate is provided on the anode part of a stacked layer element, so that the element damages at the time of connecting to a lead frame can be reduced. According to the method of JP-A-9-320895, a lead frame is formed into a special shape so as to protect the element and then a stacked layer element is integrated therein. The examples shown in the figures of these patent applications have similar arrangement of an element to that in the present invention. However, the relationship in proper positioning between the lead frame and the element is not referred to and the effect thereof is not described. Furthermore, JP-A-10-144573 discloses a structure in which a projection is provided on the anode side of a lead frame and the element anode part is provided with a positioning part so as to be positioned to engage with the projection. The structure is essentially different from the present invention in that the anode part of capacitor element has a positioning part.
In the case of conventional solid electrolytic capacitors, when a lead frame composed of copper, a copper alloy or the like is bonded to the anode end part of a capacitor element, they are bonded with an electroconductive adhesive or mechanically connected by bending and caulking the terminals. Alternatively, they are bonded by welding with a lead based solder material, laser welding or the like. However, the bonding method using an electroconductive adhesive takes a long time for applying the adhesive. In particular, when a number of single plate capacitor elements are stacked and bonded, the working is very cumbersome. The mechanical bonding method by caulking the connected parts of lead frame is not suitable for those having small bonding parts and results in unstable bonding. Furthermore, in the case of welding with a lead based solder material, there is a fear that excessive lead removed from the welded part would cause problems such as environmental pollution. The bonding method by laser welding has the problem of increased costs and so forth.
In addition to these bonding methods, resistance welding of a terminal of capacitor element to a lead frame is known (JP-A-3-188614). This is to perform resistance welding using exclusively an iron nickel alloy (42 alloy) as the lead frame material. In addition, in the case where aluminum foil is used as the valve-acting metal of the capacitor element, the lead frame composed of copper, copper alloy or the like cannot be bonded by simple resistance welding. This is because resistance welding is a bonding method in which the metal at the part to be welded is molten for welding by heat generation (Joule heat) due to electric resistance and aluminum, copper, copper alloy and the like materials having high electroconductivity have low resistance so that they generate less heat. In addition because of good heat conductivity, the part to be bonded can be molten only insufficiently so that it is difficult to weld these materials.
Furthermore, among conventional solid electrolytic capacitors, those having a capacitor element bonded to a lead frame that has plating over the entire surface thereof are also known. However, when the lead frame is plated on its entire surface, superposed on the capacitor element and heat treated, it may be resulted that the plating metal is molten not only in the portion to be bonded with the capacitor element but also in the portion to be contact with the mold resin and a defect called solder ball will occur. The known structure to avoid such an inconvenience is obtained by a method of plating a copper substrate of a lead frame on the entire surface thereof with solder, removing the plating where the mold resin contacts when sealing therewith to expose the copper substrate, roughening the exposed surface, and then mounting a capacitor element on the roughened surface and bonding it to the surface (JP-A-5-21290). However, the method has problems that the amount of plating on the bonding part of the capacitor element is insufficient and that the bonding strength is low.
The present invention is intended to provide a solid electrolyte capacitor and manufacturing method thereof free of the above-mentioned problems encountered in the conventional technology.
In order to reduce impedance, the contact area between a capacitor element and the cathode part of a lead frame (cathode-side lead frame) may be made as large as possible. However, this causes an increase in the leakage current after the reflow soldering or the like. The contact area is made large for the purpose of reducing the resistance on the cathode part of a capacitor element as much as possible and in addition, for protecting the element from mechanical or thermal stress at the time of lamination of elements, anodic welding or armoring with resin.
Despite these effects, if the cathode-side end corner part of a lead frame is present in the vicinity of the boundary between the cathode part and the insulating part of an element, there is a risk that stress concentration may occur in the vicinity of the boundary due to bending stress to rupture the dielectric film. Furthermore, silver paste used for bonding the lead frame to the element may enter from the boundary between the insulating part and the cathode electrically conducting layer directly into the vicinity of the dielectric film, giving rise to occurrence of short circuit at fine portions and increase in the leakage current. Because of these, the leakage current increases after the reflow soldering or the yield in the inspection process decreases.
Therefore, an object of the present invention is to provide a solid electrolytic capacitor having low impedance and high reliability by constructing the capacitor to enable relaxation of the thermal stress generated at the reflow soldering or the like and thereby establishing a method of preventing the increase of leakage current.
Another object of the present invention is to provide a solid electrolytic capacitor that in relation to its bonding structure is free of defects such as solder ball at the time of bonding the capacitor element to the lead frames by welding and also has excellent bonding strength.
Still another object of the present invention is to provide a solid electrolytic capacitor that in relation to the bonding structure of the anode part thereof enables bonding of the anode end part of the capacitor element to the lead frame by resistance welding, facilitates the work, gives high bonding strength and causes no environmental pollution or the like.
In order to attain the above-described object, the solid electrolytic capacitor of the present invention is constructed such that the contact area between a capacitor element and the cathode part of a lead frame is reduced while preventing the increase of resistance at the cathode part, namely, the length of a lead frame on the cathode side is reduced to a predetermined dimension, and a predetermined space is provided between the insulating part of the capacitor element and the lead frame to prevent the endmost part of the lead frame on the cathode side from approaching to the insulating part of the capacitor element, so that the stress concentration of the element around there can be mitigated and the excess silver paste for bonding can be prevented from entering from the vicinity of the boundary of the insulating part into the neighborhood of the dielectric layer.
Furthermore, in order to mitigate the stress concentration of the element in the vicinity of the endmost part of the lead frame on the cathode side, the endmost part of the lead frame is chamfered so that it can have a rounded structure. The increase of series resistance can be dealt with, for example, by eliminating the window part of the lead frame. In this instance, the position of the bonding the capacitor element to the lead frame is important. Accordingly, the lead frame is marked by half etching or a laser ray, so that the bonding position can be confirmed exactly.
As such actions being taken, the dielectric layer is scarcely subject to microscopic destructions by mechanical or thermal stress, and as a result the yield is improved and the leakage current does not increase after the reflow soldering or the like.
The term xe2x80x9ccircumferentially providexe2x80x9d as used herein means to provide by winding it around a certain member. The term xe2x80x9cto placexe2x80x9d as used herein shows not only the vertical relationship in the space but also includes the state where the matter to place and the material on which the matter is placed are disposed so as to contact each other. The term xe2x80x9cbondedxe2x80x9d as used herein means that two parts are connected and joined.
More specifically, the present invention provides the following solid electrolytic capacitor and method for manufacturing the solid electrolytic capacitor.
(1) A solid electrolytic capacitor comprising:
a capacitor element comprising a substrate comprising a valve-acting metal having a dielectric film layer on the surface thereof, the substrate having end parts, an anode part assigned to one end part of said substrate, an insulating part comprising an insulating layer of a predetermined width provided circumferentially on the substrate in contact with said anode part, and a cathode part comprising a solid electrolytic layer and an electroconducting layer stacked in sequence on said dielectric film layer over an area other than said anode part and said insulating part,
an anode-side lead frame bonded to said anode part and a cathode-side lead frame bonded to said cathode part of said capacitor element, and
a resin sealing that seals said capacitor element,
wherein the capacitor element and the lead frames are bonded such that the end part of said insulating layer on the side of said cathode part and the endmost part of said cathode-side lead frame in said capacitor element are spaced apart at a distance.
(2) The solid electrolytic capacitor as described in (1) above, wherein the distance between the end part of said insulating layer on the side of said cathode part and the endmost part of said cathode-side lead frame is from {fraction (1/40)} to xc2xd of the length of the cathode part.
(3) The solid electrolytic capacitor as described in (1) or (2) above, wherein said solid electrolytic capacitor is a stacked layer capacitor element comprising a plurality of said capacitor elements stacked and bonded one on another.
(4) The solid electrolytic capacitor as described in any one of (1) to (3) above, wherein the endmost part of said lead frame has a chamfered surface.
(5) The solid electrolytic capacitor as described in any one of (1) to (4) above, wherein said lead frame has at least one mark that indicates a position for placing and bonding said single or stacked layer capacitor element.
(6) The solid electrolytic capacitor as described in any one of (1) to (5) above, wherein said lead frames have no window part in areas thereof that contact said cathode or anode part of said capacitor element.
(7) The solid electrolytic capacitor as described in any one of (1) to (6) above, wherein said lead frame comprises a copper-based material or a material whose surface is plated with a copper-based or zinc-based material.
(8) A method for manufacturing a solid electrolytic capacitor, comprising the steps of:
assigning an anode part to an end part of a substrate comprising a valve-acting metal having on the surface thereof a dielectric film layer and circumferentially providing an insulating layer having a predetermined width on said substrate in contact with said anode part;
providing an electrolytic layer on said dielectric film layer over an area other than said anode part and said insulating part and stacking an electroconducting layer on said electrolytic layer to form a cathode part, to thereby fabricate a capacitor element; and
bonding lead frames to said anode part and cathode part, respectively, of said capacitor element such that the end part on the side of said cathode of said insulating layer and the endmost part of said lead frame on the side of said cathode part of said capacitor element are spaced apart at a distance.
(9) The method for manufacturing a solid electrolytic capacitor as described in (8) above, wherein the distance between the end part of said insulating layer on the side of said cathode part and the endmost part of said cathode-side lead frame is from {fraction (1/40)} to xc2xd of the length of the cathode part.
(10) The method for manufacturing a solid electrolytic capacitor as described in (8) or (9) above, wherein said lead frame is bonded to a stacked layer capacitor element comprising a plurality of said capacitor elements stacked one on another.
Furthermore, the solid electrolytic capacitor of the present invention has increased bonding strength and is free of defects such as solder ball by bonding the capacitor element and the lead frame by welding as follows. That is, in a portion to be sealed with a resin, the surface of the lead frame that contacts the mold resin is not plated but low melting point metal plating is performed only in the portion where the lead frame contacts the capacitor element to bond the lead frame and the capacitor element.
Moreover, in relation to bonding between the anode end part of the capacitor element and the lead frame, the solid electrolytic capacitor of the present invention bonds the anode side lead frame and the capacitor element utilizing a dielectric film on the surface of valve-acting metal exposed on the anode end part. That is, low melting point metal plating is performed on the surface of the lead frame, the anode end part of the capacitor element is superposed on the thus plated part, and resistance welding is performed. Then the plating metal is molten due to resistance heat generation of the dielectric film exposed on the anode end part and bonding is effected. Performing such a bonding allows the low melting point metal on the surface of the lead frame to be molten because of resistance heat generation and dissolved into the dielectric film on the surface of the valve-acting metal on the end part of the capacitor element. As a result, the anode part of the capacitor element and the lead frame are firmly and integrally bonded. In the case where a stack of capacitor elements is used, the dielectric films on the surface of the stacked anode end parts are molten due to the resistance welding to dissolve into each other so that the bonding strength is improved. Therefore, the anode end part of the capacitor element can be bonded to the lead frame with ease and high reliability. In particular, electrochemically formed aluminum foil, valve-acting metal, can be bonded to a good electroconducting lead frame composed of copper, a copper alloy based material or the like with high reliability. Moreover, use of a plating metal containing no or a less amount of lead, lead alloy or the like, for example, use of undercoat plating of nickel with surface plating of tin, can prevent environmental pollution due to lead and the like problems.
That is, the present invention further provides the following solid electrolytic capacitor and method for manufacturing the solid electrolytic capacitor:
(11) A solid electrolytic capacitor comprising:
a capacitor element comprising a substrate comprising a valve-acting metal having a dielectric film layer on the surface thereof, the substrate having end parts, an anode part assigned to one end part of said substrate, an insulating part comprising an insulating layer of a predetermined width provided circumferentially on the substrate in contact with said anode part, and a cathode part comprising a solid electrolytic layer and an electroconducting layer stacked in sequence on said dielectric film layer over an area other than said anode part and said insulating part,
an anode-side lead frame bonded to said anode part and a cathode-side lead frame bonded to said cathode part of said capacitor element, and
a resin sealing that seals said capacitor element,
wherein in said resin sealed area, lead frame surfaces where the resin contacts have no plating and only lead frame surfaces where said lead frames contact said capacitor element have low melting temperature metal plating that bonds said lead frames and said capacitor element to each other.
(12) A solid electrolytic capacitor comprising:
a capacitor element comprising a substrate comprising a valve-acting metal having a dielectric film layer on the surface thereof, the substrate having end parts, an anode part assigned to one end part of said substrate, an insulating part comprising an insulating layer of a predetermined width provided circumferentially on the substrate in contact with said anode part, and a cathode part comprising a solid electrolytic layer and an electroconducting layer stacked in sequence on said dielectric film layer over an area other than said anode part and said insulating part,
an anode-side lead frame bonded to said anode part and a cathode-side lead frame bonded to said cathode part of said capacitor element, and
a resin sealing that seals said capacitor element,
wherein a lead frame surface on the side of said anode has low melting temperature metal plating, and said anode-side end part of said capacitor element is placed and bonded to said plating by resistance welding utilizing resistance heat in said dielectric film layer.
(13) The solid electrolytic capacitor as described in (11) or (12) above, wherein a surface on the anode-side lead frame has low melting temperature metal plating and said anode-side end part of said capacitor element is placed and bonded to said plating by resistance welding, and wherein a surface on the cathode-side lead frame is bonded to said cathode part such that an end part on the side of said cathode of said insulating layer and an endmost part of said lead frame on the side of said cathode part of said capacitor element are spaced apart at a distance.
(14) The solid electrolytic capacitor as described in any one of (11) to (13) above, wherein said valve-acting metal is a material selected from aluminum, tantalum, titanium, niobium and alloys thereof.
(15) The solid electrolytic capacitor as described in any one of (11) to (14) above, wherein said lead frame comprises copper or a copper alloy or a copper-based material or a material having plated on the surface thereof a copper-based material or a zinc-based material.
(16) The solid electrolytic capacitor as described in any one of (11) to (15) above, wherein said low melting temperature metal plating comprises a metal or alloy having a melting temperature lower than that of said valve-acting metal and wherein said plating layer has a thickness of 0.1 to 100 xcexcm.
(17) The solid electrolytic capacitor as described in any one of (11) to (16) above, wherein said low melting temperature metal plating comprises undercoat nickel plating and surface tin plating.
(18) The solid electrolytic capacitor as described in any one of (11) to (17) above, wherein the bonding position of said lead frame is on a central part or circumferential surface of said stacked layer capacitor element.
(19) A method for manufacturing a solid electrolytic capacitor, comprising the steps of:
assigning an anode part to an end part of a substrate comprising a valve-acting metal having on the surface thereof a dielectric film layer and circumferentially providing an insulating layer having a predetermined width on said substrate in contact with said anode part;
providing an electrolytic layer on said dielectric film layer over an area other than said anode part and said insulating part and stacking an electroconducting layer on said electrolytic layer to form a cathode part, to thereby fabricate a capacitor element; and
bonding lead frames to said anode part and cathode part, respectively, of said capacitor element such that only a lead frame surface where said lead frame contacts said capacitor element is plated with a low melting temperature metal to bond said lead frame and said capacitor element to each other.
(20) A lead frame comprising a material selected from copper or copper alloy, a material having on the surface thereof plating comprising a copper-based material or a zinc-based material and having low melting temperature metal plating only a portion in a resin sealed area that is to be contacted with a capacitor element.